The present invention relates to an organic electroluminescent device. More particularly, the present invention relates to a thin film device which emits light when an electric field is applied to an organic luminescent layer comprising an organic compound.
The thin film type electroluminescent (EL) device that has heretofore been used has an inorganic material such as a II-VI group compound semiconductor (e.g., ZnS, CaS and SrS) doped with Mn or rare earth element (e.g., Eu, Ce, Tb, Sm) as an emission center. An EL device prepared from the foregoing inorganic material has the following disadvantages:
1) AC driving is required (50 to 1,000 Hz);
2) A high driving voltage is required (approx. 200 V);
3) The full color arrangement is difficult (particularly for blue color); and
4) The cost of peripheral driving circuit is high.
In recent years, however, an EL device comprising an organic thin layer has been developed to solve the foregoing problems. In particular, in order to enhance the luminous efficiency, the type of electrodes to be used has been optimized for the purpose of enhancing the efficiency of injection of carrier from the electrodes. Further, an organic electroluminescent device comprising a hole transport layer made of an aromatic diamine and an organic luminescent layer made of aluminum complex of 8-hydroxyquinoline has been developed (Appl. Phys. Lett., vol. 51, page 913, 1987). As a result, the luminous efficiency has been drastically enhanced as compared with conventional EL device comprising a single crystal such as anthracene. Further, it has been practiced to dope an aluminum complex of 8-hydroxyquinoline as a host material with a laser fluorescent dye such as coumarin (J. Appl. Phys., vol. 65, page 3610, 1989), thereby enhancing the luminous efficiency or convert the wavelength of emitted light. Thus, these products have been provided with properties close to practical properties.
Besides the foregoing electroluminescent devices comprising low molecular weight materials, an electroluminescent device comprising, as a luminescent layer material, a polymer such as poly(p-phenylenevinylene), poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] and poly(3-alkylthiophene) and a device comprising a mixture of a polymer such as polyvinyl carbazole with a low molecular weight light-emitting material and an electron-transporting material have been developed.
These organic electroluminescent devices are disadvantageous in that they have a low driving stability and require a high driving voltage.
In particular, the lowering of the required driving voltage is one of great problems which must be solved when the organic electroluminescent device is applied to the area of flat panel display. For example, when the organic electroluminescent device is used as a display device for portable devices, a lower consuming power is a particularly great point. Further, when the organic electroluminescent device is applied to small-sized character display device, a simple matrix driving method is mainly employed. In this method, however, the device is required to emit light at a high duty ratio and a high luminance in an extremely short period of time. Thus, a high driving voltage is required, lowering the power emission efficiency.
On the other hand, specific examples of instability during driving include lowering of luminance, rise of voltage during constant current driving, and occurrence of non-emission area (dark spot). There are some causes for these instabilities. It is considered that these instabilities are attributed to deterioration of cathode material, particularly the interface on the organic luminescent layer side thereof. In other words, in the case of organic electroluminescent device, a low work function metal such as magnesium alloy and calcium is used as a cathode to facilitate the injection of electron from the cathode into the organic layer. However, these metals can be easily oxidized due to the moisture in the air and thus become a great cause for instabilities during driving. In other words, a cathode containing a low work function metal is effective for the lowering of the required driving voltage of device but can easily undergo oxidative deterioration and thus needs improvements.
As a cathode material there has been recently used a cathode containing an alkaline metal instead of metal electrode such as magnesium-silver alloy, which has been used at an early stage. This is because that the use of an alkaline metal having a low work function makes the difference from LUMO (lowest empty level) of the organic layer to be small, increasing the amount of electron to be injected into the organic layer and hence making it possible to drive at a low voltage. Thus, an aluminum-lithium alloy is often used at present. However, the content of lithium in aluminum cannot be easily controlled. Further, the aluminum-lithium alloy has a stability problem. Moreover, aluminum, which is a main material of the cathode, has a higher work function than alkaline metal or alkaline earth metal and is relatively stable. Thus, aluminum is often used as a cathode material. However, aluminum can be easily oxidized in the air to become aluminum oxide and lacks storage stability. Thus, the use of silver, which is stabler, as a cathode is under study.
In order to use, as a cathode material, silver which has a high work function and is hardly injected into the organic layer, there have heretofore been proposed a method of vacuum-evaporating Li alone onto an organic compound layer to a thickness as extremely thin as about 10 angstrom, and then depositing silver thereon (IEEE Trans. Electron Devices., vol. 40, page 1342, 1993), a method of depositing silver as a cathode on 4,4xe2x80x2-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (TPD) or 4,4xe2x80x2-biscarbazolebiphenyl (Cz-TPD) as a cathode buffer layer (Synthetic Metals, vol. 91, page 195, 1997), etc.
However, the two-layer type cathode is limited in the thickness of Li layer and thus cannot perform when the thickness of Li layer is not smaller than 20 angstrom. Therefore, the two-layer type cathode cannot be easily prepared. The method involving the use of TPD or Cz-TPD as a cathode buffer layer is disadvantageous in that the buffer layer itself has a small electron mobility and the required driving voltage must be raised. Thus, this method is not sufficient with respect to the luminous efficiency.
Thus, in order to drive the device at a low voltage, it is necessary that the amount of electron to be injected from the cathode into the organic luminescent layer be increased. On the other hand, in order to prepare a stable device having a high luminous efficiency, it is desirable that silver, which has an excellent stability as cathode material, be used. Accordingly, the study of further improvement of device structure and material for enhancing the efficiency of injection of electron from the cathode made of silver into the organic luminescent layer has been desired.
The present invention was accomplished under these circumstances. An object of the present invention is to provide an organic electroluminescent device which can emit light at a low voltage and a high efficiency and can be driven in a stable manner.
The gist of the present invention resides in an organic electroluminescent device comprising an anode, an organic luminescent layer, an electron transport layer and a cathode formed in this order on a substrate, wherein the electron transport layer comprises an electron-transporting material and an alkaline metal, at least a part of said alkaline metal is dispersed in the electron transport layer in the form of cation and the cathode comprising silver or a silver alloy; as well as a process for the preparation thereof.
The inventors made extensive studies in order to provide an organic electroluminescent device which can emit light at a low voltage and a high efficiency and can be driven in a stable manner. As a result, it was found that the above objects can be achieved by providing an electron transport layer between the organic luminescent layer and the silver or a silver alloy cathode and using a specific material in the electron transport layer.
The electron transport layer is formed by a compound which can efficiently transport electron injected from the cathode toward the organic luminescent layer between the electrodes across which an electric field is applied. The electron transport layer has an effect of preventing from exciton produced by recombination in the organic luminescent layer from being diffused and quenched in the cathode.
The electron-transporting material to be used in the electron transport layer is required to be a material which can accept electron injected from the cathode at a high efficiency, exhibits a high electron mobility and thus can efficiently transport electron. As materials satisfying these requirements there have been proposed metal complexes such as aluminum complex of 8-hydroxyquinoline. However, these electron-transporting materials generally have a lower charge mobility than hole-transporting materials.
In the present invention, the electron transport layer comprises an alkaline metal incorporated therein in the form of cation in combination with such an electron-transporting material to drastically enhance its charge mobility and receptivity of electron from the cathode. Thus, an organic electroluminescent device can be realized which exhibits an improved efficiency of injection of electron from the cathode comprising silver or a silver alloy, which is highly stable, into the organic luminescent layer, an excellent stability during driving, requires a low driving voltage and exhibits a high luminous efficiency.
In the present invention, a phenanthroline derivative such as bathophenanthroline or a metal complex such as aluminum complex of 8-hydroxyquinoline as the electron-transporting material of the electron transport layer to realize excellent film properties.
Further, by providing a hole blocking layer having an ionization potential which is higher than that of the organic luminescent layer by a factor of 0.1 eV or more between the organic luminescent layer and the electron transport layer, the luminous efficiency of the organic electroluminescent device can be further enhanced.
As the material of the hole blocking layer, it is preferable to use a metal complex, styryl compound, triazole derivative or phenanthroline derivative. The thickness of the hole blocking layer is preferably from 0.5 nm to 50 nm.
The process for the preparation of the organic electroluminescent device of the invention is a process for the preparation of an organic electroluminescent device comprising a step of co-evaporating an alkaline metal obtained by reducing a compound containing an alkaline metal and an electron-transporting material during vacuum deposition to form an electron transport layer. In this manner, an electron transport layer can be efficiently formed which comprises an electron-transporting material and an alkaline metal and has at least a part of the alkaline metal dispersed therein in the form of cation to exhibit remarkably excellent electron-transporting properties.